Conventional refrigeration systems utilize a recirculating refrigerant for removing heat from a low temperature side of the refrigeration system and for discharging heat at a high temperature side of the refrigeration system. The work input necessary to operate the refrigeration system is provided by a motor driven compressor which receives low pressure gaseous refrigerant and compresses it to a high pressure. This high pressure gaseous refrigerant is supplied to a condenser where heat is removed from the gaseous refrigerant to condense it to a liquid. This liquid refrigerant is then supplied through an expansion valve to an evaporator wherein heat is transferred from a heat transfer fluid to the liquid refrigerant to evaporate the liquid refrigerant. The heat transfer fluid is thereby cooled and then used to cool a load, such as to cool a building. The evaporated refrigerant from the evaporator is returned to the compressor for recirculation through the refrigeration system.
The amount of heat absorbed by the refrigerant liquid in the evaporator includes the heat of vaporization of the liquid refrigerant; that is, the amount of heat which must be absorbed by a liquid at a given temperature to convert it to a gas at the same temperature. In addition, the gaseous refrigerant in the evaporator may absorb additional heat which raises its temperature above the temperature of vaporization. Such gaseous refrigerant is said to be superheated, and the amount by which the temperature of the gaseous refrigerant is raised above the vaporization temperature is expressed in degrees of superheat.
The expansion valve and its control system play important parts in overall refrigeration system efficiency and in control of superheat. In many applications of refrigeration systems it is desired to prevent appreciable superheating of the gaseous refrigerant or to regulate the superheat of the refrigerant to a preselected magnitude. Also, in certain applications, to provide ideal operating conditions, the expansion valve should admit an amount of refrigerant that can be evaporated and just slightly superheated in the evaporator. That is, the evaporator should be "wetted" with liquid refrigerant along approximately its entire length to provide heat transfer conditions which maximize the operating efficiency of the refrigeration system.
Normally, regardless of the particular application, the expansion valve and its control system operate to prevent flow of liquid refrigerant from the evaporator to the compressor of the refrigeration system. Also, normally, the expansion valve is designed to move to its fully closed position in response to certain malfunctions detected in the refrigeration system. This is done to protect against liquid refrigerant "flooding" the compressor which may cause damage to the compressor. However, as a result of certain failures, such as the expansion valve itself sticking open, flooding of the compressor is still possible. Also, as a result of certain other failures in the refrigeration system, such as loss of refrigerant charge, a stuck closed expansion valve, or a plugged filter-dryer through which liquid refrigerant flows from the condenser to the expansion valve, too little refrigerant may flow through the evaporator to the compressor causing the temperature of the gaseous refrigerant entering the compressor to reach a dangerously high level which may cause damage to the compressor or have other adverse consequences for the refrigeration system.